Conventional miniature motors using a rotor with three or more salient poles and a commutator with three or more grooves have no self inactivating function. For this reason, when excess load is applied to the motor or rotation is forcibly discontinued, the motor tends to burn out due to overheating because excess current is kept flowing, or even if stopped temporarily, the motor happens to restart suddenly immediately after the load is removed. As a result, the miniature motor, when applied to a model airplane, for example, tends to burn out if the model airplane falls to the ground, or the plane tends to run out of control due to the sudden restart of the motor. Furthermore, an electric fan to which the miniature motor having no self inactivating function is incorporated is very dangerous if a child happens to put his finger into the fan guard.
To equip a miniature motor with a self-inactivating function, the motor should be of such a construction that current supply be interrupted when the rotor of the motor is at a spontaneous-stop angular position. With a miniature motor having two salient rotor poles, where it is relatively easy to limit the spontaneous-stop angular position, current supply can be easily interrupted at the limited stop position.
The term "self-inactivating function" used herein means a function to prevent current from flowing in the motor which is forcibly stopped operation while the power is on by means of a specially designed insulating spacer on the commutator. The motor having the self-inactivating function requires an external force (by hand, for example) to be started again.
The term "spontaneous stop" used herein means that after the power is turned off, the motor keeps running for a while due to inertial force and eventually comes to a halt.
FIG. 11 shows a rotor of a conventional type of miniature motor using a two salient rotor poles. FIG. 12 is a cross-sectional view taken substantially along line XXII--XXII in FIG. 11. In these figures, numeral 7 refers to a rotor shaft, 8 to a salient rotor pole core, 10 to a commutator, 11 to a flange of the pole core 8, and 16 to a pole winding, respectively. Since this miniature motor has a two-pole stator (not shown) consisting of two magnets, the angular positions at which the motor stops spontaneously due to the influence of cogging are only two positions; namely, an angular position at which one salient rotor pole faces the N pole of the stator while the other salient rotor pole faces the S pole, and an angular position, rotated 180 degrees from the aforementioned angular position, at which one salient rotor pole faces the S pole of the stator while the other salient rotor pole faces the N pole. Conventional miniature motors having a self-inactivating function have such a construction that current supply is interrupted when the rotor is at any of the spontaneous-stop angular positions.
FIG. 13 is an exploded perspective view of a commutator in the rotor of the conventional construction shown in FIG. 11. Two commutator segments 12 are provided on the outer peripheral surface of a commutator cylinder 14 made of an insulating resin, with two insulating spacers 15, which are integrally formed with the commutator cylinder 14, provided between the two commutator segments 12 to separate and insulate the commutator segments. Numeral 13 refers to a commutator support ring. Two brushes are disposed in such a positional relationship that the brushes are each on the two insulating spacers 15 when the motor rotor is at any of the spontaneous-stop angular positions. With this arrangement, the self-inactivating function of a miniature motor having a two-pole rotor can be accomplished as current supply is interrupted at any of the positions where the rotor is stopped, and no sudden restart is caused.
To obtain a larger torque without increasing the size of the motor, the motor is required to have three or more salient rotor poles. It has been practically impossible to achieve a self-inactivating function with the conventional motors having three or more rotor poles. Since the motor commutator is originally used to feed current to rotor windings, it is not desirable to increase the torque of the motor by providing a large number of insulating spacers on the commutator surface to increase the non-conduction angle at which no current is supplied to the rotor. Motors with three or more rotor poles, on the other hand, involve multiple spontaneous-stop angular positions. In a three-pole motor, for example, in which the motor stops at positions of 360.degree./6, it is practically impossible to interrupt current supply at all the stop positions to maintain motor torque.
FIG. 14 is a diagram of assistance in explaining stop positions in a conventional type of miniature motor having a three-pole rotor. At position (a) of the spontaneous-stop angle of 0.degree. at the left of FIG. 14, a first salient rotor pole core faces the S pole of the stator. This represents a spontaneous-stop angular position. At a position (b), rotated 60.degree. from the original position, a second salient rotor pole faces the N pole of the stator, where the motor stops spontaneously. At a position (c), further rotated 60.degree. from the original position, a third salient rotor pole faces the S pole of the stator where the motor stops spontaneously. Similarly, there are spontaneous-stop angular positions at every turn by 60.degree.. Since the 360.degree. position (g) is the same as the 0.degree. position, there are a total of six spontaneous-stop angular positions. To achieve the self-inactivating function, insulating spacers must be inserted in all the three gaps between the three commutator segments to interrupt current supply at all the six spontaneous-stop angular positions. As described above, however, it is practically impossible to achieve this in terms of motor torque. Even when only one insulating spacer is inserted to interrupt current supply at the spontaneous-stop angular position (a) of 00 to prevent the motor from restarting suddenly, current supply cannot be interrupted at the next position (b), rotated 60.degree., or at the position (c), etc. The terms "Can be started" and "Cannot be started" in FIG. 14 mean this state.